Luminescence Properties of an Orthorhombic KLaF4 Phosphor Doped with Pr3+ Ions under Vacuum Ultraviolet and Visible Excitation

Fluorides have a wide bandgap and therefore, when doped with the appropriate ions, exhibit emissions in the ultraviolet C (UVC) region. Some of them can emit two photons in the visible region for one excitation photon, having a quantum efficiency greater than 100%. In a novel exploration, praseodymium (Pr3+) ions were introduced into KLaF4 crystals for the first time. The samples were obtained according to a high-temperature solid-state reaction. They exhibited an orthorhombic crystal structure, which has not been observed for this lattice yet. The optical properties of the material were investigated in the ultraviolet (UV) and visible ranges. The spectroscopic results were used to analyze the Pr3+ electronic-level structure, including the 4f5d configuration. It has been found that KLaF4:Pr3+ crystals exhibit intense luminescence in the UVC range, corresponding to multiple 4f → 4f transitions. Additionally, under vacuum ultraviolet (VUV) excitation, distinct transitions, specifically 1S0 → 1I6 and 3P0 → 3H4, were observed, which signifies the occurrence of photon cascade emission (PCE). The thermal behavior of the luminescence and the thermometric performance of the material were also analyzed. This study not only sheds light on the optical behavior of Pr3+ ions within a KLaF4 lattice but also highlights its potential for efficient photon management and quantum-based technologies.


Introduction
In recent years, the exploration of rare-earth-doped crystals has significantly advanced the realm of photonics and optical technologies.Praseodymium (Pr), a member of the lanthanide series, intrigues researchers with its distinctive spectroscopic properties despite possessing only two 4f electrons.Notably, phosphors activated by Pr 3+ ions have undergone extensive investigation across diverse fields, such as light sources [1][2][3], optical thermometry [4][5][6], laser technologies [7,8], glass filters [9,10] and bioimaging [11].The optical properties of these kinds of materials in the UV range are particularly interesting because they can be modulated by changing the crystallographic surroundings of the activator ion.This is possible due to their 4f 5d levels, which have an energy of about 61,580 cm −1 [12] in free Pr 3+ ions.However, upon doping into crystals, these levels are split and shifted towards lower energies.This red shift significantly depends on the crystal structure type and the material composition hosting the Pr 3+ ions.Consequently, under excitation using UV light, three distinct scenarios emerge [13]: • If E(4f 5d) < E( 1 S 0 ), only interconfigurational 4f 5d → 4f transitions are observed; • If E(4f 5d) >> E( 1 S 0 ), nonradiative relaxation between these two levels occurs, and only 4f → 4f transitions are observed; • If E(4f 5d) ≥ E( 1 S 0 ), both 4f 5d → 4f and 4f → 4f transitions take place.
The process observed in the second case is known as 'photon cascade emission' (PCE) or 'quantum cutting' (QC).In this phenomenon, the absorption of one high-energy photon is followed by a two-step emission process that generates two photons.As a consequence, a quantum efficiency higher than 100% is possible to obtain.PCE was initially observed independently by Sommerdijk et al. [18] and Piper et al. [19] in 1974, marking a groundbreaking discovery.Since then, phosphors exhibiting PCE have gained a lot of attention due to their possible application in lamps based on Xe discharge, competitive with Hgdischarge-based lamps [20].In Pr 3+ -doped materials, PCE is observed as two characteristic transitions, 1 S 0 → 1 I 6 (around 400 nm) and 3 P 0 → 3 H 4 (around 480 nm), which can be detected after excitation to the 4f 5d level.Because E(4f 5d) > E( 1 S 0 ) is the crucial condition for PCE occurrence, this process is highly dependent on the type of crystal lattice.Thus, photon cascade emission was reported for limited types of hosts, including fluorides and oxides such as YF 3 [21], KMgF 3 [22], LiSrAlF 6 [23], SrAl 12 O 19 [24], and LaMgB 5 O 10 [25].
Crystals represented by the formula ALnF 4 (A = Li + , Na + , K + , Ln = Y 3+ , La 3+ , Lu 3+ ) have been studied as a promising host for RE 3+ ions due to their low phonon energies, high optical transparency in the UV-visible range, and good chemical and photochemical stability.Among these fluorides, some doped with Pr 3+ ions have exhibited noteworthy characteristics.For instance, LiLuF 4 :Pr 3+ and LiYF4:Pr 3+ show intense UVC luminescence, attributed to interconfigurational 4f 5d → 4f transitions, while others like NaYF 4 :Pr 3+ and KYF 4 :Pr 3+ demonstrate luminescence due to the PCE process [26].It is worth noting that KLaF 4 crystals stand out due to their exceptionally low phonon energy (262 cm −1 ) [27], positioning them as ideal hosts for highly efficient upconverters and downconverters [28].Because of their great mono dispersion, long luminescence lifetime, and high up-or downconversion efficiency, they may have versatile and promising applications as luminescent nano-biolabels.Recent work by Deo et al. reported KLaF 4 nanoparticles co-doped with Eu 3+ , Er 3+ , and Yb 3+ ions, allowing simultaneous excitation in the visible and NIR regions, resulting in upconversion and downconversion emissions concurrently [29].This dualmode approach presents valuable applications in bioimaging or information encryption.Additionally, Nd 3+ -doped KLaF 4 nanoparticle colloidal solutions were proposed as NIR high-power liquid laser materials and amplifiers [30].Despite extensive studies on KLaF 4 , the incorporation of Pr 3+ ions into this fluoride matrix remains unexplored.Consequently, the optical properties of KLaF 4 :Pr 3+ are still unknown.
In this work, KLaF 4 powders doped with Pr 3+ ions were synthesized for the first time.To prepare this material, a high-temperature solid-state reaction was applied, leading to an orthorhombic structure.To the best of our knowledge, KLaF 4 crystals with this type of space group have not been reported yet since this fluoride usually crystallizes in the cubic or hexagonal phase [27].Furthermore, we studied the optical properties of KLaF 4 doped with different concentrations of Pr 3+ ions at room and low temperatures, as well as the temperature dependence of luminescence and thermometric performance.A special focus was put on spectroscopic measurements in the deep UV range, which revealed the great potential of this system as a photon cascade emitter.
To obtain X-ray powder diffraction (XRPD) patterns, a Panalytical X'Pert PRO powder diffractometer with a copper K α radiation source (λ = 1.54056Å) was used.The morphology, composition, and mapping of the samples were investigated using an FE-SEM FEI Nova NanoSEM 230 (FEI Company, a part of Thermo Fisher Scientific, Waltham, MA, USA) equipped with an energy-dispersive X-ray spectrometer, the EDAX Genesis XM4.The SEM images were recorded at 5.0 kV in beam deceleration mode, which improves imaging parameters such as the resolution and contrast.In the case of the SEM-EDX measurements, a large area (250 µm × 200 µm) of the samples was scanned at 20 kV.The powder samples were included in the carbon resin (PolyFast Struers, Ballerup, Denmark) and then pressed using an automatic mounting press CitoPress-1 (Struers, Ballerup, Denmark) in order to obtain a large and flat area.Signals from three randomly selected areas were collected to ensure satisfactory statistical averaging.The particle size distribution histogram was calculated using ImageJ v1.53k software by collecting the size of 109 particles.
To examine the excitation and emission spectra in the UVC range, a VUV McPherson spectrometer equipped with a water-cooled deuterium lamp and a Hamamatsu photomultiplier R955P was utilized.An FLS1000 fluorescence spectrometer from Edinburgh Instruments, equipped with a xenon lamp, was employed for the excitation and emission spectra, as well as the decay profiles in the visible range.The same spectrometer connected to the Linkam THMS 600 Heating/Freezing Stage was used to perform temperature-dependent measurements.

Structure
The structure of the KLaF 4 crystals has been studied and described in numerous papers [28,[31][32][33]; however, only cubic and hexagonal types of lattices have been reported.In this work, new orthorhombically ordered microcrystals were synthesized via a hightemperature solid-state reaction.Figure 1a shows the XRPD spectra of pure and Pr 3+ -doped KLaF 4 .The XRPD patterns were indexed similarly to the orthorhombic structure of KCeF 4 (SG: Pnma; ICSD file No. 23229 [34]), and no peaks corresponding to any other phase were observed.In this lattice, La 3+ ions are coordinated by nine fluorine atoms, forming a so-called tricapped triangular prism (Figure 1b).The unit cell parameters of KLaF 4 :Pr 3+ are provided in Table S1.Notably, the lattice parameters and unit cell volume decrease with an increase in the activator concentration due to the smaller radius of Pr 3+ ions (1.179 Å for CN = 9) compared to that of La 3+ ions (1.216 Å for CN = 9) [35].
morphology, composition, and mapping of the samples were investigated using an FE-SEM FEI Nova NanoSEM 230 (FEI Company, a part of Thermo Fisher Scientific, Waltham, MA, USA) equipped with an energy-dispersive X-ray spectrometer, the EDAX Genesis XM4.The SEM images were recorded at 5.0 kV in beam deceleration mode, which improves imaging parameters such as the resolution and contrast.In the case of the SEM-EDX measurements, a large area (250 µm × 200 µm) of the samples was scanned at 20 kV.The powder samples were included in the carbon resin (PolyFast Struers, Ballerup, Denmark) and then pressed using an automatic mounting press CitoPress-1 (Struers, Ballerup, Denmark) in order to obtain a large and flat area.Signals from three randomly selected areas were collected to ensure satisfactory statistical averaging.The particle size distribution histogram was calculated using ImageJ v1.53k software by collecting the size of 109 particles.
To examine the excitation and emission spectra in the UVC range, a VUV McPherson spectrometer equipped with a water-cooled deuterium lamp and a Hamamatsu photomultiplier R955P was utilized.An FLS1000 fluorescence spectrometer from Edinburgh Instruments, equipped with a xenon lamp, was employed for the excitation and emission spectra, as well as the decay profiles in the visible range.The same spectrometer connected to the Linkam THMS 600 Heating/Freezing Stage was used to perform temperature-dependent measurements.

Structure
The structure of the KLaF4 crystals has been studied and described in numerous papers [28,[31][32][33]; however, only cubic and hexagonal types of lattices have been reported.In this work, new orthorhombically ordered microcrystals were synthesized via a hightemperature solid-state reaction.Figure 1a shows the XRPD spectra of pure and Pr 3+doped KLaF4.The XRPD patterns were indexed similarly to the orthorhombic structure of KCeF4 (SG: Pnma; ICSD file No. 23229 [34]), and no peaks corresponding to any other phase were observed.In this lattice, La 3+ ions are coordinated by nine fluorine atoms, forming a so-called tricapped triangular prism (Figure 1b).The unit cell parameters of KLaF4:Pr 3+ are provided in Table S1.Notably, the lattice parameters and unit cell volume decrease with an increase in the activator concentration due to the smaller radius of Pr 3+ ions (1.179 Å for CN = 9) compared to that of La 3+ ions (1.216 Å for CN = 9) [35].2c), the mean size of the grains was estimated to be 6.9 µm.The observed particle size and morphology are characteristic of the solidstate method of synthesis, and similar results were also obtained for LuPO 4 powders prepared using the same synthesis method [36].Figure 2d shows the energy-dispersive spectroscopy (EDS) spectrum for a KLaF 4 :1.5%Pr3+ sample.Emission peaks were observed at 0.8 and 4.7 keV for lanthanum, 3.3 keV for potassium, 0.7 keV for fluorine, and 5.1 eV for praseodymium.The inset in Figure 2d displays the weight and atomic percentages of the elements.The atomic percentage corresponds to the ratio of the stoichiometric value of the element in the formula to the sum of the stoichiometric values of all the elements, which confirms that the KLaF 4 crystals were obtained with a new crystallographic structure.Furthermore, EDS mapping analysis confirmed the homogeneous distribution of the K, La, F, and Pr elements (refer to Figure S1 in the Supplementary Materials).
thesized particles have an agglomerated, non-uniform shape with sharp edges.Based on the size distribution histogram (Figure 2c), the mean size of the grains was estimated to be 6.9 µm.The observed particle size and morphology are characteristic of the solid-state method of synthesis, and similar results were also obtained for LuPO4 powders prepared using the same synthesis method [36].Figure 2d shows the energy-dispersive spectroscopy (EDS) spectrum for a KLaF4:1.5%Pr3+ sample.Emission peaks were observed at 0.8 and 4.7 keV for lanthanum, 3.3 keV for potassium, 0.7 keV for fluorine, and 5.1 eV for praseodymium.The inset in Figure 2d displays the weight and atomic percentages of the elements.The atomic percentage corresponds to the ratio of the stoichiometric value of the element in the formula to the sum of the stoichiometric values of all the elements, which confirms that the KLaF4 crystals were obtained with a new crystallographic structure.Furthermore, EDS mapping analysis confirmed the homogeneous distribution of the K, La, F, and Pr elements (refer to Figure S1 in the Supplementary Materials).

VUV Excitation
Figure 3a presents the excitation spectrum of the KLaF4:1%Pr 3+ powder monitored at 272 nm.To describe all the observed transitions, the experimental data were deconvoluted

Optical Properties 3.2.1. VUV Excitation
Figure 3a presents the excitation spectrum of the KLaF 4 :1%Pr 3+ powder monitored at 272 nm.To describe all the observed transitions, the experimental data were deconvoluted into five Gaussian-like components (blue solid lines in Figure 3), which can correspond to the transition from the 3 H 4 ground state of Pr 3+ ions to higher-energy-lying 5d levels.The energies and Full Width at Half Maximum (FWHM) values of the detected bands are detailed in Table S2.The broad band, commencing around 71,000 cm −1 , could be associated with host absorption, though other potential origins cannot be excluded.Further detailed research is necessary to confirm the assignment of this band.
into five Gaussian-like components (blue solid lines in Figure 3), which can correspond to the transition from the 3 H4 ground state of Pr 3+ ions to higher-energy-lying 5d levels.The energies and Full Width at Half Maximum (FWHM) values of the detected bands are detailed in Table S2.The broad band, commencing around 71,000 cm −1 , could be associated with host absorption, though other potential origins cannot be excluded.Further detailed research is necessary to confirm the assignment of this band.The 5d-level positions of the lanthanide ions are influenced by factors such as centroid shift (εc), crystal field splitting (εcfs), and redshift (D(A)) [14].Dorenbos investigated these parameters across various crystalline lattices, including fluorides [14], chlorides [15], oxides [16], and aluminates [17].
The centroid shift (εc) represents the energy difference between the average positions of the 5d levels in a free RE 3+ ion and within a crystalline host.The εc value is influenced by the coordinating ligand and is the smallest for fluoride matrices according to the nephelauxetic series: Crystal field splitting (εcfs) is the energy difference between the lowest and highest 5d components.It tends to increase with a decreasing coordination number.In the orthorhombic KLaF4 lattice, La 3+ ions are coordinated by nine fluorine anions, resulting in small εcfs (11,592 cm −1 ).Both εc and εcfs contribute to the redshift (D(A)) of the first allowed 4f → 5d transition in host A. This redshift can be expressed as: where E5d(free) is the position of the lowest 5d level of RE 3+ as a free ion (for Pr 3+ , E5d(free) = 61,580 cm −1 ) and E5d(A) is the energy of the lowest 5d level for RE 3+ ions doped into compound A (E5d(KLaF4) = 52,880 cm −1 ).For Pr 3+ in KLaF4, D(KLaF4) is calculated as 8700 cm −1 , aligning with the data for other fluoride crystals [12].Although the values of εc, εcfs, and D(A) reported in Refs.[12,[14][15][16][17] were calculated for Ce 3+ ions, Dorenbos suggests these parameters are similar for all Ln 3+ ions when doped in the same host compound [37].
Considering the influence of the crystalline environment on the position of the Pr 3+ 5d levels, fluoride matrices emerge as suitable candidates for the PCE process due to the relatively high energy of the lowest 5d level.Since the first allowed 4f → 5d transition in The 5d-level positions of the lanthanide ions are influenced by factors such as centroid shift (ε c ), crystal field splitting (ε cfs ), and redshift (D(A)) [14].Dorenbos investigated these parameters across various crystalline lattices, including fluorides [14], chlorides [15], oxides [16], and aluminates [17].
The centroid shift (ε c ) represents the energy difference between the average positions of the 5d levels in a free RE 3+ ion and within a crystalline host.The ε c value is influenced by the coordinating ligand and is the smallest for fluoride matrices according to the nephelauxetic series: Crystal field splitting (ε cfs ) is the energy difference between the lowest and highest 5d components.It tends to increase with a decreasing coordination number.In the orthorhombic KLaF 4 lattice, La 3+ ions are coordinated by nine fluorine anions, resulting in small ε cfs (11,592 cm −1 ).Both ε c and ε cfs contribute to the redshift (D(A)) of the first allowed 4f → 5d transition in host A. This redshift can be expressed as: where E 5d (free) is the position of the lowest 5d level of RE 3+ as a free ion (for Pr 3+ , E 5d (free) = 61,580 cm −1 ) and E 5d (A) is the energy of the lowest 5d level for RE 3+ ions doped into compound A (E 5d (KLaF 4 ) = 52,880 cm −1 ).For Pr 3+ in KLaF 4 , D(KLaF 4 ) is calculated as 8700 cm −1 , aligning with the data for other fluoride crystals [12].Although the values of ε c , ε cfs , and D(A) reported in Refs.[12,[14][15][16][17] were calculated for Ce 3+ ions, Dorenbos suggests these parameters are similar for all Ln 3+ ions when doped in the same host compound [37].
Considering the influence of the crystalline environment on the position of the Pr 3+ 5d levels, fluoride matrices emerge as suitable candidates for the PCE process due to the relatively high energy of the lowest 5d level.Since the first allowed 4f → 5d transition in KLaF 4 :Pr 3+ has an energy of 52,880 cm −1 , the lowest 5d level must be located above the 1 S 0 (E( 1 S 0 ) ≈ 47,000 cm −1 ) [38]. Figure 3b depicts the emission spectrum of KLaF 4 :1%Pr 3+ recorded under 160 nm excitation.Seven bands were observed at 216, 237, 252, 272, 338, 405, and 484 nm.They all exhibit significantly smaller FWHM values, as indicated in Table S2, in comparison to the bands observed in the excitation spectrum.This implies that they align with 4f → 4f transitions.The first six bands can be assigned to transitions from the 1 S 0 level to the 3 H 4 , 3 H 6 , 3 F 3 , 1 G 4 , 1 D 2 , and 1 I 6 states, respectively, while the last band is considered to be 3 P 0 → 3 H 4 .The experimental branching ratios (β ex ) of the transitions from the 1 S 0 level were calculated as the ratio between the area under the specific peak and the area of the spectrum in the 22,500-55,000 cm −1 range.The obtained results are listed in Table S2.It could be noted that the highest values were observed for the 1 S 0 → 1 G 4 and 1 S 0 → 1 I 6 transitions due to their spin-allowed character.According to Kück [39], the typical branching ratio of 1 S 0 → 1 I 6 transitions varies from 60 to 80% depending on the host type.Here, a β ex ( 1 S 0 → 1 I 6 ) of 39.8% is much smaller than the expected value.This discrepancy is attributed to the significant influence of instrumental factors, such as the efficiency of the photomultiplier and the specifications of the diffraction grating, on the experimental branching ratio.The measured spectrum was corrected for both of these factors.However, even if correction is performed, the real relative intensities of the observed transitions can remain unknown.
A schematic illustration of the PCE process observed in KLaF 4 :Pr 3+ under VUV excitation is presented in Figure 4.After the absorption of one 160 nm photon, Pr 3+ ions are excited into a 4f 5d configuration and then relax nonradiatively into the lower-lying 1 S 0 level.During radiative relaxation into the ground state, the emission of two photons occurs at 406 ( 1 S 0 → 1 I 6 ) and 484 nm ( 3 P 0 → 3 H 4 ).Because the lowest 5d level is located about 6000 cm −1 above 1 S 0 , 5d → 4f transitions are not observed.
orded under 160 nm excitation.Seven bands were observed at 216, 237, 252, 272, 338, 405, and 484 nm.They all exhibit significantly smaller FWHM values, as indicated in Table S2, in comparison to the bands observed in the excitation spectrum.This implies that they align with 4f → 4f transitions.The first six bands can be assigned to transitions from the S0 level to the 3 H4, 3 H6, 3 F3, 1 G4, 1 D2, and 1 I6 states, respectively, while the last band is considered to be 3 P0 → 3 H4.The experimental branching ratios (βex) of the transitions from the S0 level were calculated as the ratio between the area under the specific peak and the area of the spectrum in the 22,500-55,000 cm −1 range.The obtained results are listed in Table S2.It could be noted that the highest values were observed for the 1 S0 → 1 G4 and 1 S0 → 1 I6 transitions due to their spin-allowed character.According to Kück [39], the typical branching ratio of 1 S0 → 1 I6 transitions varies from 60 to 80% depending on the host type.Here, a βex ( 1 S0 → 1 I6) of 39.8% is much smaller than the expected value.This discrepancy is attributed to the significant influence of instrumental factors, such as the efficiency of the photomultiplier and the specifications of the diffraction grating, on the experimental branching ratio.The measured spectrum was corrected for both of these factors.However, even if correction is performed, the real relative intensities of the observed transitions can remain unknown.
A schematic illustration of the PCE process observed in KLaF4:Pr 3+ under VUV excitation is presented in Figure 4.After the absorption of one 160 nm photon, Pr 3+ ions are excited into a 4f5d configuration and then relax nonradiatively into the lower-lying 1 S0 level.During radiative relaxation into the ground state, the emission of two photons occurs at 406 ( 1 S0 → 1 I6) and 484 nm ( 3 P0 → 3 H4).Because the lowest 5d level is located about cm −1 above 1 S0, 5d → 4f transitions are not observed., the metal-ligand (ML) distance is smaller for the 5d electronic configuration than for the 4f.

Visible Excitation
As KLaF4 crystals doped with Pr 3+ ions are introduced in this paper for the first time, an exploration of their optical properties in the visible range was undertaken.Figure 5a presents the excitation spectrum of KLaF4:0.5%Pr3+ monitored at 608 nm.Three 4f → 4f transitions, 3 H4 → 3 P2, 3 P1, and 3 P0, were observed at 444, 468, and 480 nm, respectively.[40].and Mahlik et al. [41], the metal-ligand (ML) distance is smaller for the 5d electronic configuration than for the 4f.

Visible Excitation
As KLaF 4 crystals doped with Pr 3+ ions are introduced in this paper for the first time, an exploration of their optical properties in the visible range was undertaken.Figure 5a presents the excitation spectrum of KLaF 4 :0.5%Pr3+ monitored at 608 nm.Three 4f → 4f transitions, 3 H 4 → 3 P 2 , 3 P 1 , and 3 P 0 , were observed at 444, 468, and 480 nm, respectively.The band at 484 nm corresponds to the transition from the first excited Stark sublevel of the 3 H 4 state, which is thermally populated at room temperature.Upon exciting the sample with 444 nm light, emissions from the 3 P 1 and 3 P 0 levels were detected (Figure 5b).The most intense bands observed at 484, 608, 640, and 720 nm correspond to the spin-allowed transition from 3 P 0 to the 3 H 4 , 3 H 6 , 3 F 2 , and 3 F 4 levels, respectively [42].
The band at 484 nm corresponds to the transition from the first excited Stark sublevel of the 3 H4 state, which is thermally populated at room temperature.Upon exciting the sample with 444 nm light, emissions from the 3 P1 and 3 P0 levels were detected (Figure 5b).The most intense bands observed at 484, 608, 640, and 720 nm correspond to the spin-allowed transition from 3 P0 to the 3 H4, 3 H6, 3 F2, and 3 F4 levels, respectively [42].To investigate the influence of the dopant concentration, emission spectra were acquired under 444 nm excitation for samples with varying Pr 3+ ion concentrations but under the same measurement conditions (Figure S2).The highest relative intensity of visible luminescence was obtained for the sample doped with a 0.5% content (see inset in Figure 5b).For a higher Pr 3+ content, concentration quenching of the luminescence occurs.The process responsible for concentration quenching is cross-relaxation (CR), which depends on the distance between the ions involved in these phenomena.As the concentration increases, the average distance between the activator ions is shortened, leading to a rise in the effectiveness of CR.The two main CR processes responsible for quenching the Pr 3+ luminescence can be described using the following equations: (2) To investigate the influence of the dopant concentration, emission spectra were acquired under 444 nm excitation for samples with varying Pr 3+ ion concentrations but under the same measurement conditions (Figure S2).The highest relative intensity of visible luminescence was obtained for the sample doped with a 0.5% content (see inset in Figure 5b).For a higher Pr 3+ content, concentration quenching of the luminescence occurs.The process responsible for concentration quenching is cross-relaxation (CR), which depends on the distance between the ions involved in these phenomena.As the concentration increases, the average distance between the activator ions is shortened, leading to a rise in the effectiveness of CR.The two main CR processes responsible for quenching the Pr 3+ luminescence can be described using the following equations: The energies of the Stark levels used in Equations ( 2) and (3) were obtained from Table S3.Following the cross-relaxation (CR) process, there is a slight energy mismatch (as is ev-ident in Equations ( 2) and ( 3)), which can be easily compensated by the host phonon energies.Consequently, it can be assumed that both processes are practically resonant at room temperature.Moreover, concentration quenching significantly influences the lifetime of the emitting levels.Figure 5c,d show the emission decay curves of the 3 P 0 and 1 D 2 levels, respectively, measured for different activator ion contents.Only the sample with the lowest dopant concentration (0.1%) exhibited an exponential decay profile at both levels.When the concentration is increased, the decay patterns become increasingly non-exponential.To calculate the average observed decay time τ OB , a bellowed equation was applied [43]: where I(t) and I(0) indicate the luminescence intensity at time t and t = 0, respectively.The obtained results are listed in Table 1.The luminescence of both levels exhibits the longest decay time for the smallest concentration of Pr 3+ ions (37 µs for 3 P 0 and 346 µs for 1 D 2 ).The lifetime of the 1 D 2 -level emissions decreases more rapidly with the concentration of Pr 3+ than that of the 3 P 0 level.In the CR described using Equation ( 2), spin-forbidden transitions are involved, while the one occurring from level 1 D 2 is spin-allowed, making it more efficient.
Table 1.Radiative decay times (τ R ), observed decay times (τ OB ), and cross-relaxation rates (W CR ) of the 3 P 0 and 1 D 2 levels for samples with different Pr 3+ concentrations.Since the observed luminescence decay is a result of the radiative and nonradiative relaxation paths, the observed transition rate (W OB ) can be described as: where W R and W NR are the rates of the radiative and nonradiative transitions, respectively.There are two main processes involved in the nonradiative relaxation of ions: multiphonon relaxation (MNR) and CR.Due to the small phonon energies of KLaF 4 and the large energy difference between the 3 P 0 and 1 D 2 (4034 cm −1 ) and 1 D 2 and 1 G 4 levels (7081 cm −1 ), the MNR process has a neglected influence on the 3 P 0 and 1 D 2 decay times.According to this, W NR is equal to W CR , which is the rate of the cross-relaxation processes: where τ R is the radiative decay time.
For the calculation of radiative lifetimes, the Judd-Ofelt theory proves invaluable.Independently proposed by Brian Judd [44] and George S. Ofelt [45] in 1962, this approach facilitates the determination of the Einstein coefficient A aJ,bJ for the spontaneous emission of electric dipole transitions, and subsequently the radiative lifetime τ R = 1/A aJ,bJ , using the formula: S aJ : bJ ′ .( 7) Here e, h, λ, and n represent the electron charge, Planck's constant, transition wavelength, and refractive index, respectively.J denotes the total angular momentum of the initial state, while S signifies the line strength of the electric dipole transition between the two J (spin-orbit) multiplets a and b.
The line strength within the Judd-Ofelt theory is expressed as follows: with summation over all the components 2J + 1.The aJ U t bJ ′ ⟩ terms represent the reduced-matrix elements of the matrix, wherein the transition values between each level of a rare-earth ion are calculated, independent of the surrounding environment.These were calculated and tabulated by Carnal [46].Ω λ (λ = 2, 4, 6) are the phenomenological parameters linked to the host.These parameters are determined using fitting by comparing the experimental oscillator strengths measured for as many transitions in the absorption spectrum as possible with theoretical ones, expressed as: where f ed denotes the electric dipole oscillator strength, with the other constants as previously explained.This classical approach, although highly successful [47], is restricted to monocrystals and glasses, as it necessitates measuring the absorption spectra using transmission techniques, along with the exact dopant concentration and refractive index of the host.A thorough introduction to this theory by Robert D. Peacock is recommended for interested readers [48].
Towards the end of the 20th century, Brazilian colleagues proposed a useful method for calculating the Ω λ parameters for Eu 3+ based on emission spectra [49,50].They observed that for several transitions, only one aJ U t bJ ′ ⟩ parameter differs from zero, enabling straightforward calculation of Ω λ .This approach paved the way for further innovation, such as utilizing excitation spectra, as demonstrated by W. Luo et al. [51]; diffuse reflection spectra, as explored by Gao et al. [52]; or fluorescence decay analyses, as verified by M. Luo et al. [53].In this study, we employed a method proposed by Ćirić et al. [54] to determine the Ω λ parameters from the emission spectra.
With the Ω λ parameters determined, it becomes feasible to calculate the Einstein coefficient of spontaneous emission, radiative decay times, emission branching ratios, and the rates of nonradiative transitions.These parameters were calculated by us using the web application published by Ćirić et al., and their values were equal to 0.488, 1.186, and 4.659 [all in 10 −20 cm 2 ] for Ω 2 , Ω 4 , and Ω 6 , respectively.Thus, it was possible to calculate the radiative lifetimes of the 3 P 0 and 1 D 2 levels, which are 42 and 582 µs, respectively, taking n = 1.6 as the refractive index and data from the 300 K emission spectra.In our view, this was justified for a primary reason, it ensured compliance with the assumptions of the Judd-Ofelt theory regarding the equal occupancy of the 4f electronic configuration levels.Therefore, using the classical method, absorption spectrum data obtained at room temperature should be utilized for calculations.In his study, Ćirić et al. suggest measurements at 77 K to avoid the presence of the 1 D 2 → 3 H 4 transition in the spectrum.However, in the case of fluoride, the lattice vibrations are minimal, and as a result, this transition is not observed at low dopant concentrations.
Since cross-relaxation is a process within a pair of ions, W CR increases with increases in the Pr 3+ concentration for both the 3 P 0 and 1 D 2 levels.The order of magnitude of the CR rate is the same for both levels when comparing samples with the same dopant content.However, it is important to note that CR rates are not absolute values.To compare the CR rates of the two processes, it is appropriate to normalize them to the rates of the radiative transitions of the levels involved.This approach allows not only the comparison of different CR processes within a matrix but also the comparison of CR processes occurring in different matrices.Figure 6 illustrates the dependence of the W CR /W R ratio on the function of Pr 3+ ion concentration.One can see that the effectiveness of the CR process occurring from level 1 D 2 is enormous compared to the CR arising from 3 P 0 , being for the 2% sample 14 times more effective at the depopulation of the 1 D 2 level than the radiative process.

Thermal Behavior of Luminescence
The optical properties of KLaF4:Pr 3+ were investigated at different temperatures as well.Figure 7 presents the excitation and emission spectra of the KLaF4:0.5%Pr3+ powder measured at 25 K, compared with those recorded at 300 K. Assignment of all the detected peaks is showed in both spectra.As expected, at a low temperature, the spectral lines are narrower, and their multiple-component shape can be easily observed.Based on these 25 K spectra, the experimental energies of the 2S+1 Lj Stark levels were calculated and are listed in Table S3.Please note that the energies of the 3 F3, 1 G4, 1 D2, and 1 S0 levels were estimated based on the room-temperature spectra (Figures 3b and 5b) because no transitions involving those levels were observed in the low-temperature spectra.

Thermal Behavior of Luminescence
The optical properties of KLaF 4 :Pr 3+ were investigated at different temperatures as well.Figure 7 presents the excitation and emission spectra of the KLaF 4 :0.5%Pr3+ powder measured at 25 K, compared with those recorded at 300 K. Assignment of all the detected peaks is showed in both spectra.As expected, at a low temperature, the spectral lines are narrower, and their multiple-component shape can be easily observed.Based on these 25 K spectra, the experimental energies of the 2S+1 L j Stark levels were calculated and are listed in Table S3.Please note that the energies of the 3 F 3 , 1 G 4 , 1 D 2 , and 1 S 0 levels were estimated based on the room-temperature spectra (Figures 3b and 5b) because no transitions involving those levels were observed in the low-temperature spectra.

Thermal Behavior of Luminescence
The optical properties of KLaF4:Pr 3+ were investigated at different temperatures as well.Figure 7 presents the excitation and emission spectra of the KLaF4:0.5%Pr3+ powder measured at 25 K, compared with those recorded at 300 K. Assignment of all the detected peaks is showed in both spectra.As expected, at a low temperature, the spectral lines are narrower, and their multiple-component shape can be easily observed.Based on these 25 K spectra, the experimental energies of the 2S+1 Lj Stark levels were calculated and are listed in Table S3.Please note that the energies of the 3 F3, 1 G4, 1 D2, and 1 S0 levels were estimated based on the room-temperature spectra (Figures 3b and 5b) because no transitions involving those levels were observed in the low-temperature spectra.Comparing the 25 K and 300 K excitation spectra of KLaF 4 :0.5%Pr3+ (Figure 7a), the band at 484 nm, which occurs at room temperature, is not observed at a low temperature.This signal is associated with a transition from the second excited crystal field level of the ground state (here named 3 H 4 (2)) to the 3 P 0 level.According to Boltzmann's statistical law, the electron distribution between the two electron levels obeys the dependence , where N 1 and N 0 are the populations of the higher and lower levels, respectively; ∆E is the energy gap between these levels; k B represents the Boltzmann constant; and T is the temperature.Since the ∆E between 3 H 4 (0) and 3 H 4 (2) is 147 cm −1 (see Table S3), both levels are populated and participate in sample excitation at 300 K.However, when the temperature is lowered to 25 K, the thermal population of the 3 H 4 (2) level is reduced, and only transitions from the 3 H 4 (0) level are detected.A similar effect is observed in the low-and room-temperature emission spectra (Figure 7b) recorded under 444 nm excitation.At 300 K, emissions from both the 3 P 0 and 3 P 1 levels are observed, but at 25 K, transitions from the 3 P 1 level are quenched.
To better understand the thermal behaviour of Pr 3+ luminescence in KLaF 4 crystals, emission spectra under 444 nm excitation were recorded in the 85-760 K temperature range (Figure 8), with a 25 K interval.The observed trend reveals thermal quenching of luminescence occurs while the sample temperature is increased, which is also presented in the inset.Considering the whole measured temperature range, the I(T) function exhibits a complex nature and cannot be fitted using one simple formula.This is because integrated luminescence includes the intensities of multiple transitions, which are characterized by different thermal behaviours.Generally, to fit I(T) dependence, the Arrhenius equation is used: where I(T) is the luminescence intensity measured at temperature T, I(0) is the initial intensity, C means constant, and ∆E is the activation energy of the thermal quenching.
In Figure 9a, the plot depicts the dependence of ln    Recently, a distinct luminescence thermometry strategy based on the emission of praseodymium-doped phosphors has been elaborated.Among others, down-converted emissions from the two thermally coupled excited states 3 P0,1 of Pr 3+ were recorded in a temperature range of 293-593 K for YAG:Pr 3+ [55].This investigation revealed a remarkable temperature sensitivity, reaching up to 0.0025 K −1 at 573 K. Additionally, A.S. Rao observed the different temperature dependence of four various emission bands corresponding to 3 P0 → 3 H4, 3 P1 → 3 H4, 1 D2 → 3 H4, and 3 P0 → 3 F2 praseodymium transitions [56].
Consequently, four fluorescence intensity ratio models based on the relationships between different emission peaks were examined, and finally a maximum relative sensitivity was found to be 1.03% K −1 .Previously, Pr 3+ -doped tungstate phosphors were examined as well by Ruoshan Lei et al. [57], and a similar evaluation strategy resulted in quite a high relative sensitivity (1~3.25%K −1 ) and low temperature uncertainty (0.15-0.5 K) within a wide temperature range.Another exploration focused on the emission intensity variation in 3 P0 → 3 H4 and 1 D2 → 3 H4 at lower temperatures in YPO4:Pr 3+ nanopowders [58].The obtained values for the maximum absolute and relative sensitivities were 4.60 × 10 −3 K −1 at 100 K and 2.30% K −1 at 10 K, respectively.Additionally, glass materials have been The ∆E for the thermal quenching of 3 P 0 → 3 H 4 luminescence is determined to be 650 cm −1 within the temperature range of 235 K to 585 K. Notably, this value closely aligns with the energy difference between the 3 P 0 and 3 P 1 levels, calculated to be 622 cm −1 (see Table S3).As a consequence, the quenching of the 3 P 0 emission should be explained by the thermal population of the 3 P 1 level when the temperature is raised from 235 to 585 K.Moreover, analyzing the thermal behaviour of the 3 P 1 → 3 H 4 transition (Figure 9b), it has been noticed that the luminescence intensity increases in the 85-510 K range, confirming that the 3 P 1 level is populated by thermal activation.Increasing the temperature creates an additional channel for the depopulation of the 3 P 0 level via CR: Recently, a distinct luminescence thermometry strategy based on the emission of praseodymium-doped phosphors has been elaborated.Among others, down-converted emissions from the two thermally coupled excited states 3 P 0,1 of Pr 3+ were recorded in a temperature range of 293-593 K for YAG:Pr 3+ [55].This investigation revealed a remarkable temperature sensitivity, reaching up to 0.0025 K −1 at 573 K. Additionally, A.S. Rao observed the different temperature dependence of four various emission bands corresponding to 3 P 0 → 3 H 4 , 3 P 1 → 3 H 4 , 1 D 2 → 3 H 4 , and 3 P 0 → 3 F 2 praseodymium transitions [56].
Consequently, four fluorescence intensity ratio models based on the relationships between different emission peaks were examined, and finally a maximum relative sensitivity was found to be 1.03% K −1 .Previously, Pr 3+ -doped tungstate phosphors were examined as well by Ruoshan Lei et al. [57], and a similar evaluation strategy resulted in quite a high relative sensitivity (1~3.25%K −1 ) and low temperature uncertainty (0.15-0.5 K) within a wide temperature range.Another exploration focused on the emission intensity variation in 3 P 0 → 3 H 4 and 1 D 2 → 3 H 4 at lower temperatures in YPO 4 :Pr 3+ nanopowders [58].The obtained values for the maximum absolute and relative sensitivities were 4.60 × 10 −3 K −1 at 100 K and 2.30% K −1 at 10 K, respectively.Additionally, glass materials have been employed in the development of luminescence thermometers.Notably, Maturi et al. recently presented Yb 3+ /Pr 3+ co-doped fluoride phosphate glasses working as primary thermometers, demonstrating a relative thermal sensitivity and uncertainty of 1.0% K −1 and 0.5 K, respectively [59].
In relation, we observed the different effect of temperatures of 80-750 K on several emission bands of praseodymium originating in the 3 P 0 and 3 P 1 multiplets.We utilized the fluorescence intensity ratio (FIR) expressed using the equation: where ∆E is the energy gap between the two thermally coupled levels and B is the temperature-independent constant.Figure 10 displays the effect of temperature (80-750 K) on the fluorescence intensity ratio corresponding to certain praseodymium emission bands.A reliable fit in applying Equation ( 13) was achieved for ∆E = 567 cm −1 .It follows from these plots that the fluorescence intensity ratios attributed to the thermally coupled levels 3 P 0 and 3 P 1 rise with an increasing temperature, reaching their highest values at 750 K.
The variations in the absolute and relative FIR changes S A and S R with temperature are expressed as [60]: employed in the development of luminescence thermometers.Notably, Maturi et al. recently presented Yb 3+ /Pr 3+ co-doped fluoride phosphate glasses working as primary thermometers, demonstrating a relative thermal sensitivity and uncertainty of 1.0% K −1 and 0.5 K, respectively [59].
In relation, we observed the different effect of temperatures of 80-750 K on several emission bands of praseodymium originating in the 3 P0 and 3 P1 multiplets.We utilized the fluorescence intensity ratio (FIR) expressed using the equation: where ΔE is the energy gap between the two thermally coupled levels and B is the temperature-independent constant.Figure 10 displays the effect of temperature (80-750 K) on the fluorescence intensity ratio corresponding to certain praseodymium emission bands.A reliable fit in applying Equation ( 13) was achieved for ΔE = 567 cm −1 .It follows from these plots that the fluorescence intensity ratios attributed to the thermally coupled levels 3 P0 and 3 P1 rise with an increasing temperature, reaching their highest values at 750 K.The variations in the absolute and relative FIR changes SA and SR with temperature are expressed as [60]: and The potential application of optical material as a luminescence thermometer can be assessed utilizing these SA and SR parameters, which determine the thermosensitive The potential application of optical material as a luminescence thermometer can be assessed utilizing these S A and S R parameters, which determine the thermosensitive phosphor properties.In the case of Pr 3+ -doped KLaF 4 , the most efficient relative temperature sensitivities were found to be 1.70% K −1 at T = 140 K and 1.45% K −1 at T = 175 K for the ( 3 P 1 → 3 H 4 / 3 P 0 → 3 H 4 ) and ( 3 P 1 → 3 H 5 / 3 P 0 → 3 H 5 ) transitions, respectively.These values are compared with those reported for other Ln 3+ -based luminescence thermometers in Table S4 [61][62][63][64][65][66][67][68].

Figure 1 .
Figure 1.(a) X-ray powder diffraction patterns of KLaF4:Pr 3+ particles and the orthorhombic KCeF4 standard data (ICSD file No. 23229) [34].(b) Crystal structure of KLaF4 (orthorhombic system with the Pnma space group) along the a axis.

Figure
Figure 2a,b present SEM images of the KLaF 4 grains at two different scales.The synthesized particles have an agglomerated, non-uniform shape with sharp edges.Based on the size distribution histogram (Figure2c), the mean size of the grains was estimated to be 6.9 µm.The observed particle size and morphology are characteristic of the solidstate method of synthesis, and similar results were also obtained for LuPO 4 powders prepared using the same synthesis method[36].Figure2dshows the energy-dispersive

Figure 2 .
Figure 2. (a,b) SEM images of KLaF4 grains at two different scales.(c) Particle size distribution histogram.The purple line refers to the normal distribution function with µ = 6.9 µm and σ = 3.3 µm.(d) EDS spectrum of the KLaF4:1.5%Pr3+ sample; inset shows weight and atomic percentages of elements in the matrix.

Figure 2 .
Figure 2. (a,b) SEM images of KLaF 4 grains at two different scales.(c) Particle size distribution histogram.The purple line refers to the normal distribution function with µ = 6.9 µm and σ = 3.3 µm.(d) EDS spectrum of the KLaF 4 :1.5%Pr3+ sample; inset shows weight and atomic percentages of elements in the matrix.

Figure 3 .
Figure 3. (a) Excitation spectrum of the KLaF4:1%Pr 3+ sample monitored at 272 nm.The arrow indicates the excitation energy used for emission measurement and the numbers 1-5 refer to peak numbers listed in Table S2 (b) Emission spectrum of the KLaF4:1%Pr 3+ sample measured under 160 nm excitation.Both spectra were recorded at T = 300 K.

Figure 3 .
Figure 3. (a) Excitation spectrum of the KLaF 4 :1%Pr 3+ sample monitored at 272 nm.The arrow indicates the excitation energy used for emission measurement and the numbers 1-5 refer to peak numbers listed in Table S2 (b) Emission spectrum of the KLaF 4 :1%Pr 3+ sample measured under 160 nm excitation.Both spectra were recorded at T = 300 K.

Figure 4 .
Figure 4. Schematic illustration of the PCE process in KLaF4 doped with Pr 3+ ions.Solid arrows represent radiative transitions and dashed arrows correspond to nonradiative ones.Other observed transitions from the 1 S0 level were not included in the graph.Please note that according to the work of Seijo et al.[40].and Mahlik et al.[41], the metal-ligand (ML) distance is smaller for the 5d electronic configuration than for the 4f.

Figure 4 .
Figure 4. Schematic illustration of the PCE process in KLaF 4 doped with Pr 3+ ions.Solid arrows represent radiative transitions and dashed arrows correspond to nonradiative ones.Other observed transitions from the 1 S 0 level were not included in the graph.Please note that according to the work of Seijo et al.[40].and Mahlik et al.[41], the metal-ligand (ML) distance is smaller for the 5d electronic configuration than for the 4f.

Figure 6 .
Figure 6.Cross-relaxation rates (W CR ) normalized to the radiation transition rate (W R ) of 3 P 0 (red) and 1 D 2 (blue) levels for different Pr 3+ ion concentrations.

17 Figure 8 .
Figure 8. Emission spectra of the KLaF4:0.5%Pr3+ sample measured in the 85-760 K temperature range under 444 nm excitation.In the insets, the dependence of integrated luminescence intensity on temperature was plotted.

Figure 8 .
Figure 8. Emission spectra of the KLaF 4 :0.5%Pr3+ sample measured in the 85-760 K temperature range under 444 nm excitation.In the insets, the dependence of integrated luminescence intensity on temperature was plotted.

Figure 8 .
Figure 8. Emission spectra of the KLaF4:0.5%Pr3+ sample measured in the 85-760 K temperature range under 444 nm excitation.In the insets, the dependence of integrated luminescence intensity on temperature was plotted.